Ultra-high temperature ceramics (UHTCs), SiC-based ceramic matrix composites (CMCs) and cermets are promising high temperature materials. There is significant interest in using these types of ceramic-based materials in jet engines and other hypersonic vehicles. However, in order to do so, the materials need protective coatings to withstand the harsh high temperature oxidative environments to which they are exposed during use. In addition, in environments where these materials are exposed to sand under extreme conditions, the coatings must be resistant to attack and degradation by the sand.
A conventional method for protecting ceramic materials from oxidation is to rely upon an in-situ generated SiO2 scale at the surface of the ceramic. Such scales may be generated from SiC present in the ceramic. However, although SiO2 inhibits oxidation at high temperatures, at very high temperatures (e.g., ≧1350° C.) volatile SiO(g) can form instead, thereby degrading the oxidation-resistant properties of the scale.
Conventional coatings for protecting ceramics from attack by sand, such as calcium-magnesium-alumino-silicate (CMAS), utilize a glass overlay, a sealant or dewetting layer over an underlying thermal barrier coating. However, as a result of thermal cycling during the operation of turbine engines which use these coatings, the coatings crack and erode. Atmospheric plasma spray coatings have also been used. However, the microstructure of these coatings is characterized by microcracks, porosity and splat boundaries which render them susceptible to CMAS infiltration.